This project is a structural study of the neuronal interconnections in the ganglia between peripheral sense organs and central integrative sites in the horseshoe crab, Limulus polyphemus. Subjects to be investigated include the gustatory ganglion with its conspicuous glomeruli, a relay between several million primary chemosensory fibers and a tract continuing to the corpora pedunculata; the small ocellar ganglia, interposed between the ultraviolet-light-sensitive median eye and the central body; the lamina, which receives axons from retinula and eccentric cells of the compound eye; and the medulla, which forms the optic tract from the input of the lamina and the ventral eye. Specific points of study consist of the disposition and numerical relationships in convergent and divergent synaptic assemblies, connectivity within glomeruli, and the insertion of neurosecretory terminals into synaptic neuropil, a possible device for the regulation of circadian sensory responsiveness. The project will utilize light microscopic methods of serial sectioning and reconstruction for general anatomical orientation, but primarily electron microscopy in the analysis of neural networks and synaptic regions, buttressed by Golgi impregnations, selective straining techniques, and cobalt chloride filling of neurons and their processes to enhance the specificity of identifications. Functional interpretations will take cognizance of the wide spectrum of available physiological studies on sense organs in the horseshoe crab and will make the brain a useful model for neurophysiological exploration. The relative simplicity of the system should provide us with understanding of the processing pathways for sensory information and the circuitry responsible for rhythmic changes in sensitivity.